1. Field of the Invention
Systems, methods, and apparatus consistent with the present invention relate to magnetic tape storage. More particularly, the systems, methods and devices described here concern ensuring that the width of variable width data tracks is wider than a predetermined width when writing to a tape under extreme operating conditions. The also concern manufacturing tolerances and determining the center of such variable width data tracks in order to locate the read elements so the variable width tracks can be read.
2. Description of the Related Art
Digital tape-recording remains a viable solution for storage of large amounts of data. Digital information is recorded onto a storage medium such as a magnetic recording tape for a wide variety of purposes. For example, magnetic tape storage is assuming larger roles in data archiving, where reliable ultra high capacity storage at low cost is desired. Accurate recording of data onto the storage medium, storage density (storage capacity) of the medium, and accuracy of retrieving stored data are important considerations in recording data onto the medium.
In conventional tape drive technologies, such as LTO (Linear Tape Open), stable substrates, such as PEN (polyethylene naphthalate) and the like, are selected to minimize tape dimensional stability errors. Special servo tracks are used to position the write elements of the heads during writing, and the read elements of the heads during reading, so that the tape drive writes data tracks within specific boundaries and therefore the written data can be recovered at a later time. The servo track is pre-written during the tape manufacturing process. However, a drawback to using pre-written servo tracks is that the servo writing process introduces additional position errors. Additionally, utilizing servo tracks only allows the tape drive to observe position errors that are detected by the servo elements of the tape drive, whereas position errors experienced by a read or write head element of the tape drive during normal operation remain undetected. Accordingly, it is desirable to have a system and a method that can provide position accuracy as observed by the active data heads during writing or reading of data on the medium without having to rely on secondary transducers that detect dedicated servo tracks positioned between the data tracks.
To increase the storage capacity of a medium suitable for data archiving, more data tracks need to be recorded on the medium, with each track becoming increasingly narrower in width. Because of the narrower track widths, the tape becomes more susceptible to various head element position errors with respect to the tape, which occur on conventional tape drives. Lateral tape motion, for example, is one contributing factor to head position errors. Lateral tape motion may be caused by many factors including tape slitting variations, tension variations, imperfections in the guiding mechanism, and environmental factors. Environmental factors, such as heat and humidity, cause significant position errors during the reading and writing of data on the tape. Additionally, mechanical structure stability and manufacturing tolerance accuracy are the source of many other position errors that coalesce to limit tape capacity.
Generally, data track position miss-registration errors due to dimensional stability of the tape has been accepted as a limiting factor in achieving higher track densities and thus achieving higher data capacity per tape. Conventional tape drive technologies compensate for dimensional stability by writing the data tracks much wider than necessary, so that despite errors when writing the data tracks, the data tracks are still sufficiently wide to be accurately read. Another method to counter dimensional stability is to use even more stable tape technology such as PA (polyamid). However, this type of stable tape technology is also significantly more expensive than PEN and the like. Yet another method to counter dimensional stability errors, is to reduce the distance between head channels. However, this method increases the complexity and cost of the head. Accordingly, instead of reducing the distance between head channels, it is desirable to reliably decrease the track width of data tracks stored on a tape while using conventional tape technology. It is also desirable to increase the storage density of conventional tape such as DLT (Digital Linear Tape) or SDLT (Super Digital Linear Tape), using conventional inexpensive tape technology.
One way to improve the storage capacity of conventional tape drives that use conventional tape, is disclosed in U.S. Pat. No. 7,116,514, which is incorporated herein by reference in its entirety. This patent describes a head geometry technique and a system to overwrite a portion of a previous written track when writing a current track. This is possible in a conventional tape drive because tracks are written sequentially. Overwriting a portion of the previous track, or trimming that previous track, while ensuring that the trimmed, or residual, track maintains a sufficient width so that the residual track is still readable, can increase storage density and allow more data tracks to be written on the tape. Writing tracks in this manner can eliminate the dedicated servo tracks. In order to compensate for dimensional stability errors, so that the current track does not inadvertently trim so much of the previous track that the residual track is no longer readable, the previously written track is used as a reference track. An active read element that can function as a servo element is positioned on the previously written track, either at the edge or center of the track, and is able to read and servo by reading the previous track either continuously or at a set interval. As long as the read element can read the data of the residual track, the amount the current track trims the previous track is acceptable. In other words, as long as the read element can read the residual track, the width of the residual track is acceptable. However, specialized heads and tape drives are required to record data in this way. A method of detecting and/or calculating the lateral tape motion using conventional heads and with a higher degree of accuracy is still needed.
As the widths of the written tracks are made narrower, thereby increasing the data density of the medium, the write and read positional accuracy of data on the medium becomes critical. For example, even slight variations in the location of a written track due to dimensional stability errors become noticeable since the margin for error decreases proportionally with the width of the tracks. Environmental factors, such as humidity and temperature, can affect the physical dimensions of the tape and cause variations in the width of the written tracks. This problem is especially prevalent in multi channel/head tape drive systems.
Most modern tape drives have multiple heads for simultaneously writing multiple tracks on a tape. Each head contains at least one write element for writing track data, and at least one read element for reading track data. A write element writes data as a physical track. A group of adjacent physical tracks written by a specific write element is referred to as a physical band. The physical tracks written simultaneously by each of the write elements together constitute a logical track. A band is a group of physical bands, where each physical band corresponds to a unique write element of the tape drive. Some tape drives have two write elements and two read elements per head. The first write/read element pair is used for writing and reading when the tape is moving in a forward direction. A second write/read element pair is used for writing and reading when the tape is moving in a backward direction.
In such multi-head tape drive systems, it becomes important to consider manufacturing tolerances of the heads in addition to dimensional stability and environmental factors. Examples of such manufacturing tolerances include nonconformity in the width of each write element of a head, causing tracks written by different heads to have different widths. Additionally, the pitch, or distance between the write elements of adjacent heads can vary, causing the pitch, or distance between tracks written by the adjacent heads to vary. Similar manufacturing tolerances can affect the read elements as well. For example, since the tracks need not be much wider than necessary to read the tracks, if a read element of a first head is aligned to read a track, another read element of a second head may not be able to read its respective track due to alignment errors because of manufacturing tolerances in the pitch between the read elements of adjacent heads. The tape drive industry has been unable to solve this problem, and the industry has accepted it as an uncorrectable problem. For example, SDLT 220/320/600/SDLT 1 and LTO 1-4 all share the same basic servo off-track statistical error budget design. For these drives, the combination of wide written data tracks and narrow readers are required, as well as a very challenging head/drive design. Furthermore, these drives are limited in total cartridge capacity.
With the advances in track density for the next generation DLT, SDLT and LTO5 drives, errors due to the dimensional stability of tapes are currently believed in the industry to be uncorrectable as evident by the slow rate of improvements of tape track density. Dimensional stability is becoming a greater contributor to the position error budget even as tape servo systems have improved significantly. Various proposals to reduce the dimensional stability errors in the next generation super tape drives have been proposed for a number of years such as a much lower head core pitch, and, as discussed above, using a new tape material such as PA. Unfortunately, these proposals have not been implemented to date due to the significant increase of drive and cartridge cost over current technologies and the requirement for very tight drive manufacturing limits. Hence, there continues to be a need for a technique to correct dimensional stability errors using currently available, inexpensive tape and tape drive technology.
Additionally, the cost and capacity of modern tape drives has lagged behind disks even for archival data storage. Accordingly, there exists a need for a method of writing data to a tape to offer the higher capacity desired by customers from modern tape drives.